Abstract: We have obtained observations of a number of binary stars on the 6.5 meter MMT with adaptive optics (AO) in the L’ band, and also the 1.5 meter Kuiper Telescope, without AO, in the U, V, and I bands. Both sets of observations were carried out to obtain precise astrometric calibration for the Clio camera on the MMT telescope, which has been used for several AO planet-imaging surveys, the results of which have been presented elsewhere. Astrometric calibration was needed to determine if candidate planets were common proper motion companions of the parent star. We find that extremely precise astrometry of binaries is possible using AO instruments. We discuss the potential of AO astrometry to determine the masses of binary stars with great accuracy even when observations span 25% or less of the full orbital period. A number of challenges must be met to achieve the full potential accuracy. We explain how these can be addressed. Finally, we present measurements of a set of binary stars that may serve as a beginning for a catalog of wide, long-period binaries useful for precise calibration of AO instruments.